(1) Field of the Invention
The present invention relates to a ferroelectric memory apparatus and a control method of the same.
(2) Description of the Related Art
Ferroelectric memory apparatuses are excellent non-volatile memories which feature high speed and low power consumption, and have a long cycle life. Their potential is highly valued in the multimedia society. Recently, multimedia products which incorporate a ferroelectric memory apparatus have emerged one after another, growing in number year by year.
A ferroelectric capacitor which is an essential part of the ferroelectric memory apparatus for storing information indicates zero polarization at the origin of a polarization characteristic curve, i.e., at an applied voltage of 0 in FIG. 1 when the ferroelectric capacitor is not used. For example, when a voltage which exceeds a polarization inversion voltage “VCL” is applied to the ferroelectric capacitor at low temperatures, the polarization moves, according to the applied voltage, along a characteristic curve indicated by P1, P2, P3, and P4. When the application of the voltage is stopped, data “1” and “0” are stored by maintaining two polarization states, P2 and P4.
Also, as an intrinsic property of a ferroelectric capacitor, when no voltage is applied, at high temperatures, a polarization amount P12 is smaller than a polarization amount P2 at low temperatures, hysteresis width is decreased, a polarization inversion voltage “VCH” is lower than the polarization inversion voltage “VCL” at low temperatures, and the polarization moves along a characteristic curve indicated by P1, P12, P3, and P14, as shown in FIG. 1. In this way, the polarization inversion voltage has negative temperature characteristics.
Note that FIG. 1 shows an example of inversing the polarization of a ferroelectric capacitor at a fixed voltage regardless of temperature.
A ferroelectric capacitor once exposed to high temperatures maintains its reduced polarization amount even if it is placed at low temperatures again. Its polarization moves along a characteristic curve indicated by P11, P12, P13, and P14.
If no measure is taken against such temperature dependence of ferroelectric capacitor characteristics, data cannot be written properly or data retention characteristics will be degraded, making an assured operating temperature range narrower than other non-volatile memories. It is feared that this may hinder the spread of products equipped with a ferroelectric memory apparatus.
A design technique for a ferroelectric memory element has been proposed which provides a larger read margin in a predetermined operating temperature range (see, for example, Japanese Unexamined Patent Publication 9-231774 (pp. 5-6 and FIG. 4). Also, a technique has been proposed for taking measures against manufacturing variations and the temperature dependence of ferroelectric memories by improving data retention characteristics by means of a memory cell configuration which reduces an area, an equivalent circuit, a manufacturing method, and variable control of applied voltage pulse width suitable for using ferroelectric memory elements thus manufactured (see, for example, Japanese Unexamined Patent Publication 2002-184170 (pp. 9-26 and FIGS. 1, 3, and 14).
However, the conventional techniques described above are intended to increase yields in the manufacture of ferroelectric memories which operate properly in the assured operating temperature range (0° C. to 70° C.) of typical non-volatile memories and they are effective only in designing and manufacturing ferroelectric memory elements with desirable characteristics.
Thus, if there is a request to expand the operating temperature range of a ferroelectric memory apparatus but design margins available with conventional techniques cannot accommodate the request, it is necessary to newly develop a ferroelectric memory to fulfill the request. For example, when variably controlling the width of the voltage pulses applied to the ferroelectric memory, the use of a wide temperature range results in a larger difference between maximum and minimum pulse widths accordingly. This causes access time to the ferroelectric memory apparatus to be constrained by the maximum pulse width, making it difficult to operate the ferroelectric memory apparatus reliably within a given access time and thus affecting the operation of the entire apparatus.
This problem may be solved through development of a new ferroelectric memory by the application of conventional techniques. However, that will require development cost and time. Furthermore, any change in core size or specifications of ferroelectric memory will affect the entire apparatuses on each of which the ferroelectric memory will be mounted. Thus, it is not easy to expand an operation assured range.
Furthermore, the conventional techniques described above take no measure against temperature changes or neglect after a write/read operation, causing imprint degradation which makes it difficult to rewrite data in the ferroelectric memory. Thus, the conventional techniques cannot accommodate indoor and outdoor uses in cold regions where temperatures change severely.
FIG. 2 is a polarization characteristic curve of a ferroelectric capacitor suffering from imprint degradation. As shown in FIG. 2, the imprint degradation is a state in which polarization characteristics lose symmetry with respect to an applied voltage. As the imprint degradation progresses, it becomes difficult to rewrite information.
Incidentally, when the ferroelectric memory apparatuses are manufactured, there may be cases in which accelerated stress testing is conducted prior to shipment to artificially subject the ferroelectric memory apparatus to stressful conditions which cause some imprint degradation.
An example of a configuration for such testing is shown in FIG. 3. An external inspection system (not shown) applies voltage from a PAD 101 to accelerate various stressful conditions for a ferroelectric memory element 1 and a control unit 100 supplies the applied voltage to the ferroelectric memory element 1 and thereby accesses the ferroelectric memory element 1. Such access is performed, for example, by gradually changing time for which a voltage is applied, by performing write and read operations in various combinations, by continuing access for a long time, or the like.
After screening the ferroelectric memory apparatuses based on whether the degradation of characteristics falls within an allowable range, the external inspection system applies, to the screened conforming apparatuses, a voltage used for the characteristic recovery and polarization elimination, from the PAD 101, and thereby makes the ferroelectric memory element 1 recover from the degradation caused by the accelerated stress testing. Thus, the degradation of characteristics caused by the accelerated stress testing is eliminated in shipped products.
However, there is no conventional technique for making products recover from degradation of characteristics caused by thermal stress suffered after shipment.